Exemplary embodiments of the present invention relate to minimizing dynamic structural loads of an aircraft, which are introduced by an external excitation to the aircraft.
Dynamic structural loads are introduced to the structure of an aircraft by external excitations, e.g. due to wind, gusts, turbulences or similar influences, and by excitations due to pilot/flight control system demands. As a result of the flexible properties of the aircraft structure, such excitation may result in oscillations or vibrations that may exceed a given magnitude, so that they can be harmful to the aircraft structure, especially of frequencies in the range of natural or characteristic oscillations of the flexible aircraft structure. If it is desired to reduce the mass of the aircraft structure at high aspect ratio of wing and fuselage, it is necessary to take measures against excessive dynamic structural loads.
Atmospheric gusts excite dynamic wing loads especially in the wing roots as well as vertical accelerations critical for passenger safety. Besides maneuver loads these loads are sizing for the aircraft structure. If it is possible to reduce the loads, then a lighter wing design is possible, thereby improving fuel efficiency and reducing direct operating costs. Another advantage is the increase of passenger safety due to alleviation of vertical accelerations.
German patent document DE 198 41 632 C2 discloses a method for compensating structural oscillations of an aircraft, which are introduced by an external excitation, comprising detecting at least one body rate of the aircraft with a sensor arrangement, providing the at least one body rate to a flight controller, and producing movements of control surfaces of the aircraft to minimize the excited oscillations.
For updraft gust the first load peak can be eliminated by a rapid deflection of spoilers on the upper wing surface, see U.S. patent document US 2008/0265104 A. This approach is limited by control authority (reaction time, actuator power), by the loads that the rapid spoiler deflection introduces to the wing structure, as well as by the constraint that the second peak of the wing bending acceleration must not be increased by the spoiler deflection.
European patent document EP 1 814 006 A1 proposes the symmetric actuation of ailerons for gust load alleviation using a high pass filtered alpha probe signal as reference for the gust and an adaptive feed-forward controller. This approach solves the problems mentioned above, but is even more limited by the control authority of the ailerons.
Further apparatuses and methods for reducing vibrations due to gust are disclosed in German patent document DE 10 2008 014 236 A1 and European patent document EP 1 854 717 A1. European patent document EP 1 854 717 A1 discloses how control commands for alleviating disturbances can be generated. German patent document DE 10 2008 014 236 A1 describes an apparatus and a method for adaptive control.
The method disclosed in U.S. patent document US 2008/0265104 is considered as the closest prior art. Therein, it is suggested to just deflect spoilers (and other control surfaces respectively) for some prescribed time in order to compensate the gust loads, and then close them again. Such spoiler deflection excites the first wing bending mode (among other flexible modes). The deflection (and thus the load alleviation) is thus restricted by the second load peak.
Exemplary embodiments of the present invention provide an optimized method and apparatus for alleviation of dynamic structural loads on aircraft due to gust.
According to one aspect of the invention, the control surfaces are not just deflected, but actuated in a prescribed dynamic way in order to minimize not only the first load peak but also at least one subsequent peak.
Preferably, the first and all subsequent load peaks are minimized by the prescribed dynamic actuation of the control surfaces.
Preferably, this dynamic actuation can be triggered by an alpha probe or by vertical acceleration measurement, by LIDAR or by any combination of those.
Compared to the prior art as disclosed in U.S. patent document US 2008/0265104, the invention and/or the preferred embodiments thereof provide the advantage that the spoilers and/or other control surfaces are not just deflected and retracted after a certain prescribed time but opened and closed in a prescribed dynamic way. Thereby, the control authority of the spoilers is fully exploited.
According to a preferred embodiment, also for downdraft gusts, a delayed spoiler deflection alleviates the first positive peak of the wing bending.
An advantage of an embodiment of the invention is that peak loads, i.e. first peak and subsequent peaks induced by strong gust can be optimally alleviated, for updraft as well as for downdraft gusts with the available control authority and/or actuator bandwidth.
According to one aspect, the invention provides a method of dynamically alleviating loads generated on an aircraft by a disturbance of gust, the method comprising the steps of:
automatically detecting a disturbance due to gust on a flight of the aircraft;
when a disturbance due to gust is detected, automatically generating control commands for control surfaces; and applying the control commands to actuate the control surfaces; wherein the method further comprises: dynamically actuating the control surfaces in a prescribed dynamic way in order to minimize not only a first load peak but also at least a second load peak.
In a preferred embodiment, the method further comprises: generating control commands using a predetermined control command sequence that has been pre-determined in a previous control command sequence determining step.
It is further preferred that the control command sequence determining step comprises: determining an optimized control command sequence in a convex synthesis.
Preferably, the control command sequence determining step comprises:
determining a load response of the aircraft in response to an actuation of the control surface according to a basis function Hi, modeling the control command sequence as a linear combination of basis functions in the form of
  H  =            ∑              i        =        1            n        ⁢                  ⁢                  θ        i            ·              H        i            
wherein θi represents the ith element of a vector of variable θ, simulation a typical wind gust shape being alleviated by the control command sequence H, and determining the vector of variable θ by optimization calculation such that the load response to the typical wind gust shape is minimized.
According to an advantageous embodiment, the basis function is an impulse function and the load response is a load response in response to an impulse.
It is preferred that the optimization is performed over discrete time series, wherein Hi represents the basis function for the discrete time ti of a series of discrete times t1 . . . ti . . . tn.
According to a preferred embodiment, the method further comprises:
storing the control command sequence determined in the control command sequence determining step in a memory device, wherein the step of generating the control commands comprises reading out the memory device for generating the predetermined control command sequence.
Preferably the control commands are at least one element of the following group of control commands: elevator control commands for actuating an elevator control surface, spoiler control commands for deflecting spoiler control surfaces arranged on an upper and/or lower surface of a wing of the aircraft, and aileron control commands for actuating an aileron control surface.
According to a further aspect, the invention provides an apparatus for dynamically alleviating loads generated on an aircraft by a disturbance of gust, comprising: monitoring means for automatically detecting a disturbance due to gust; control command generating means for automatically generating control commands for an actuating of control surfaces when the disturbance due to gust is detected; wherein the control command generating means is configured to dynamically actuate the control surfaces in a prescribed dynamic way in order to minimize not only a first load peak but also at least a second load peak.
It is preferred that the control command generating means comprises a memory device containing a predetermined control command sequence.
According to a further aspect, the invention provides a control command sequence determining device for determining a control command sequence to be used as control commands in an apparatus as mentioned before, the device comprising: load response determining means configured to determine a load response of the aircraft in response to an actuation of the control surface according to a basis function Hi, modeling means configured to model the control command sequence as a linear combination of basis functions in the form of
  H  =            ∑              i        =        1            n        ⁢                  ⁢                  θ        i            ·              H        i            
wherein θi the ith element of a vector of variable θ, gust simulation means configured to simulate a typical wind gust shape being alleviated by the control command sequence H, and variable vector determining means configured determine the vector of variable θ by optimization calculation such that the load response to the typical wind gust shape is minimized.
It is preferred that the load response determining means is configured to use an impulse function as the basis function and is configured to determine a load response in response to an impulse.
It is further preferred that the device is configured to calculate the optimization over discrete time series, wherein Hi represents the basis function for the discrete time ti of a series of discrete times t1 . . . ti . . . tn.
According to one embodiment of the invention, the strength of the disturbance is determined using the methods as known from U.S. patent document US 2008/0265104 A1 and/or European patent document EP 1 814 006 A1. It is preferred that the strength of the disturbance is determined using the angle of wind attack.
According to a preferred embodiment, the invention provides a triggered L-infinity optimal gust load alleviation.